1. Field of the Invention
The subject invention relates to the press fabrics used to clothe the press sections of papermaking and similar machines. It more specifically relates to the use of special composite yarns having soluble cores in the weaving of these fabrics in order to provide them with increased void volume and longitudinal extensibility, and lower mark tendency.
2. Description of the Prior Art
The press fabrics used to clothe the press sections of papermaking machines are crucial components in the paper manufacturing process. One of their functions is to support and carry the paper product being manufactured through the presses which act by means of compression to force or squeeze water from the wet paper sheet. In this respect, the fabric serves as a conveyor belt during the manufacturing process.
The press fabrics also serve the function of finishing the surface of the paper sheet. That is, the surface of the press fabric is designed to be smooth and uniformly resilient so that, in the course of passing through the presses, a smooth, mark-free surface is imparted to the paper.
Perhaps most importantly, the press fabrics accept the large quantities of water pressed from the wet paper. In order to fill this function, there literally must be somewhere for the water to go within the body of the fabric. Accordingly, a successful press fabric design provides both a certain amount of void volume, for the temporary storage of water, and channels or pathways between the strands of yarn from which it is woven. These will allow water to pass through the fabric from the sheet-carrying side to the other side and, in the vicinity of the press nip, to flow longitudinally through the fabric away from the nip.
Contemporary press fabrics are available in a wide variety of styles designed to meet the requirements of the papermachines on which they are installed for the paper grades being manufactured thereon. Generally, they comprise a woven base fabric into which has been needled a batt of fine, nonwoven fibrous material. The base fabrics can be woven from monofilament, plied monofilament, multifilament, and like yarns and can be single-or multi-layered. Typically, the spaces between machine-direction (MD) and cross-machine direction (CD) yarns are intended to provide the requisite channels for water to flow in the plane of the fabric, and perpendicularly through the fabric, as well as to make up void volume for the temporary storage of water.
After the base fabric has been woven, the batt is needled into the fabric structure. This provides the finished fabric with a smooth, even surface very much like that of the original press felts woven from wool. As a consequence of the needling process, some batt fibers extend perpendicularly through the plane of the fabric. These fibers, and the voids formed between them, will generally allow water, pressed from the wet paper sheet in the press nip, to pass through the body of the fabric and away from the paper.
An undesirable effect of the needling process is the filling of much of the void volume within the base fabric. This makes the finished press fabric to some extent less capable of filling the functions for which it has been designed. In addition, this reduced void volume will fill more quickly with trapped wood fiber and fine particles, both of which will make the fabric less permeable to water. As a consequence, press fabric life on the papermachine will be shorter than desirable.
The present invention provides a solution for this problem by permitting the re-introduction of void volume after the needling process has been completed by removing soluble material which forms at least part of the core of the composite yarns used in the weaving of the base fabric.